What am I doing wrong? I am trying to fit a three point circle to [10,10], [30,15], [40,30]. QCAD does happily so with centre [14,36.5] and r = 26,800187. This has been independently verified (r = 26,800186566515) However, what appears on the display is a circle that seems slightly larger, and does not go through the points exactly when one zooms in. I can clamp a radial line to the circumference, which shows the same gap, although the numeric length is again given as exactly r. What am I doing wrong, or is there a numerical/scaling/display issue with circles? Could send you a screenshot if it helps. [win7 64 QCAD 3.0.5]Thanks.

You are (almost) right - in 3.0 the circle is much closer to the points, although again not exact. In my configuration it seems to be just a tad inside where it should be, but that's only visible with large zoom factors. (It's all a bit annoying as I am trying to illustrate a paper on circle fitting to survey data, where typically distances are large and angles are really small, so this effect somewhat ruins the whole point of the paper)

After more closer inspection,testing and 'extreamly close' zooming I can now reproduce this problem.

@TheEnglishPatient, further to your recent correspondence I have forwarded the details on to Andrew for him to have a closer look at this. Your observation regarding the graphical rendering of the file could be correct.

Clive,thanks for the answer and improvement. I don't want to quibble with your use of 'perfectly' but if you look really close at point 40,30 the circle was previously +0.063255 to the SE from the point, it is now -0.00210853 to the NW. So, it's an improvement of an order of magnitude, but still not quite there. But if it's a rendering issue, it might be of course just my graphic card/monitor...? Again, the reason I am harping on about this is that I am working on some rail track surveying, where you have globally large distances and radii, but locally only very small angles and deviations from straight lines - so these things unfortunately do become important and visible. In any case, we are down from the 'unworkable' to the 'a little annoying' level, so thanks for that again.Best Regards

Maybe - unfortunately I cannot test this any longer as my download has just expired. (Can I suggest that you put a crosshair on the centre of the circle instead of the yellow fill - it might it clearer)

TheEnglishPatient wrote:Maybe - unfortunately I cannot test this any longer as my download has just expired. (Can I suggest that you put a crosshair on the centre of the circle instead of the yellow fill - it might it clearer)

Rendering of arcs with very large radii seems to be off: In the picture the left long red line is an arc with radius 152,372.3484 m. It's the solution to a least-squares fit to a number of points (green) from a little program I am working on and should pass inside the grey circle (r 0.3875m). Looks like it doesn't, however the short red line is 'snapped' on the left to the arc, so QCAD 'knows' where the arc really is (calculated 0.130065 m from the centre and equal to the short red line length). So the arc is graphically shown ca.0.3m inside its true position.

Can the powers that be please look into this - it's driving me round the bend as this is crucial to the quality control of my other algorithm...